This invention relates to the hydrogenation of benzoquinone compounds. More specifically, it pertains to a process for the hydrogenation of diaminodihalo- and dialkylaminodihalobenzoquinones to produce diamino- and dialkylaminobenzenediols.
Diamino- and dialkylaminobenzenediols are useful as monomers in the preparation of polybenzoxazoles (PBO). Polybenzoxazoles can be prepared by reacting certain benzenediols with bisacids, bisacid halides, bisesters or bisnitriles. Polybenzoxazole fibers have high tensile strength and thermal stability and are desirable for military, aerospace and other applications requiring rigid materials.
The known methods for preparing the benzenediol monomers typically involve a multi-step synthesis wherein expensive reagents are required and overall yield is poor. For example, one method involves oxidative bishydroxylation of hydroquinone with hydrogen peroxide and an alkali to produce 2,5-dihydroxy-p-benzoquinone. The reaction of hydrogen chloride and methanol with the dihydroxybenzoquinone yields 2,5-dimethoxy-p-benzoquinone which is aminated with ammonia and then reduced with stannous chloride in concentrated hydrochloric acid to give 2,5-diamino-1,4-benzenediol with an overall yield of only about 20 percent. See R. Wolf, M. Okada and C. S. Marvel, J. Polymer Science, Part A, 6, 1503 (1968).
A direct reduction of 2,5-diamino-1,4-benzoquinone with stannous chloride and HCl to produce 2,5-diamino-1,4-benzenediol is described in Beilstein's Handbuch der Organischen Chemie, 4th ed., Main Work, V. 13, p. 791. This process suffers from the fact that the diaminobenzoquinone starting material is not readily commercially available but must be prepared from the cheaper hydroquinone by the multi-step process mentioned above. Other methods involve the reduction of a dihalodiaminobenzoquinone to the corresponding dihalodiaminohydroquinone without the elimination of the halogen substituents. See, e.g., U.S. Pat. No. 4,337,196; Brit. 1,130,275 and Fr. 1,544,504.
A simple economical process is needed that would provide a high yield and allow both the reduction of the benzoquinone and the elimination of the halogen substituents. Such a process would allow the effective production of diamino- and dialkylaminobenzenediol monomers for use in preparing the highly desirable polybenzoxazoles.